Highfrequency stimulation (HFS) of the subthalamic nucleus (STN) or internal segment of the globus pallidus is a clinically successful treatment for the motor symptoms of Parkinson's disease. However, the mechanisms by which HFS alleviates these symptoms are not understood. Whereas initial studies focused on HFS-induced changes in neuronal firing rates, recent studies suggest that changes in patterns of neuronal activity may correlate with symptom alleviation. We hypothesized that effective STN HFS reduces the disorder of neuronal firing patterns in the basal ganglia thalamic circuit, minimizing the pathological activity associated with parkinsonism. Stimulating leads were implanted in the STN of two rhesus monkeys rendered parkinsonian by 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP). Action potentials were recorded from neurons of the internal and external globus pallidus and the motor thalamus (ventralis anterior, ventralis lateralis pars oralis, and ventralis posterior lateralis pars oralis) during HFS that reduced motor symptoms and during clinically ineffective low-frequency stimulation (LFS). Firing pattern entropy was calculated from the recorded spike times to quantify the disorder of the neuronal activity. The firing pattern entropy of neurons within each region of the pallidum and motor thalamus decreased in response to HFS (n ≥ 18 and P ≤ 0.02 in each region), whereas firing rate changes were specific to pallidal neurons only. In response to LFS, firing rates were unchanged, but firing pattern entropy increased throughout the circuit (n ≥ 24 and P ≤ 10 -4 in each region). These data suggest that the clinical effectiveness of HFS is correlated with, and potentially mediated by, a regularization of the pattern of neuronal activity throughout the basal ganglia thalamic circuit.